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Topic: 1974 cb360 intake valve snapped (Read 917 times)

So this happened... Everything was fine and I was out on a ride about 5-6 miles into it I lost all power. Previous to that I was on the highway ~65-70mph. Got about a mile down a side road and that's when I lost power. Towed it to my dad's and found out the left cylinder was not firing. OK not a huge deal so I towed it home and set all the timing and got spark on left side. Replaced the plugs and still no fire on left side. Opened the intake tappet cover and the adjustment screw was gone not a good sign right off bat. There was also a large gouge in the tappet cover. So I took the valve/cam cover off and the offending spring was loose. I grabbed it and the whole spring assembly was not attached to the valve stem and fell right off. Upon closer inspection, the stem snapped right at the retainer groove. So my question is what else could be wrong, what else should I check, and what could have caused this? I'm OK with removing the engine to tear down further if needed. The bike has ran beautifully for 2+ years with the occasional timing adjustment and oil change. In those 2+ years I've probably only put about 500 miles on it. Always had charging issues so I couldn't ride far, found out it was the headlight bulb I was using, anyway that's besides the point. Once I fixed charger issue I rode it quite a bit this year with no issues. Just looking for help, thanks.

The tappet screw either screwed itself out (loosened) and the violent slapping of the screw against the top of the valve broke it, or it screwed itself in (tightened) and the pressure broke it. Any bending, or angular impact, of the valve stem could cause it to snap too, since there is a groove cut there for the retainers. Considering you found the screw laying in the head, I'd predict a loosening of the tappet screw is your culprit. A good tightening (not over-tightening) of the lock nut will hopefully keep it from happening again. I'd also inspect the rocker hole for roundness. If it bent and ovalled at all, then you may never get it to stay tight.

The tappet screw either screwed itself out (loosened) and the violent slapping of the screw against the top of the valve broke it, or it screwed itself in (tightened) and the pressure broke it. Any bending, or angular impact, of the valve stem could cause it to snap too, since there is a groove cut there for the retainers. Considering you found the screw laying in the head, I'd predict a loosening of the tappet screw is your culprit. A good tightening (not over-tightening) of the lock nut will hopefully keep it from happening again. I'd also inspect the rocker hole for roundness. If it bent and ovalled at all, then you may never get it to stay tight.

I guess it must have backed out. That's crazy because I haven't adjusted the valves in over a year. Haven't put the miles on it to need to adjust them and the bike ran great. Oh well I got the head off and the broken and bent valve out. The hard thing is going to find just one valve to replace it. I noticed there was some interference with the piston to the valve. A slight scuff mark. I'm not sure if it's enough to justify replacing the piston too??

I noticed there was some interference with the piston to the valve. A slight scuff mark. I'm not sure if it's enough to justify replacing the piston too??

That's most likely the issue. Could be that both things happened, but if the valve is interfering with the piston, then that certainly bent it. Nothing wrong with have the seats cut and new valves installed at this point. I don't know the history of the head, or if you did that recently, but while it's open...

After you get the new valve into place, try to wiggle it a bit. Any side-to-side play while it's in the guide means you'll want new guides, too.

I had the same issue with my 360, but mine was self-inflicted. I didn't have the rocker arms positioned correctly before I torqued down the rocker cover. One of the tappets had gotten caught in front of the valve stem instead of resting over it.